Ventricular fibrillation is a life-threatening cardiac arrhythmia which, if untreated, will most probably cause death of the patient in whom it occurs. There exists an identifiable population of patients who have a high risk of enduring ventricular fibrillation and for whom no effective medical therapy is currently available. An alternative therapy is implantation of an automatic defibrillator. Most of the defibrillators which have been proposed and are under development rely on a transvenous catheter, which contains at least one defibrillating electrode, to accomplish ventricular defibrillation. We have discovered that the defibrillating catheter imposes limits on the magnitude of current that can be delivered. The objective ofthis research program is to develop a model of the defibrillating catheter and to verify the predictions of the model in large animals.